The drive belts generally comprise an elastomeric body, in which a plurality of longitudinal filiform resistant inserts, also referred to as “cords”, is embedded, and a plurality of teeth coated by a coating fabric. Such teeth may be arranged in a longitudinal direction for multiple race or poly-V belts, or transversally for toothed belts.
Each component of the belt contributes to increasing the performance in terms of mechanical resistance, so as to decrease the risk of failure of the belt and to increase the specific transmissible power.
The coating fabric increases the resistance to abrasion and hence protects the working surface of the belt from wear due to rubbing between the sides and the tops of the belt teeth and the sides and the bottoms of the races of the pulley with which the belt interacts.
Furthermore, the coating fabric reduces the coefficient of friction on the working surface, reduces the deformability of the teeth and especially reinforces the root of the tooth thereby avoiding the failure thereof.
The cords especially contribute to ensuring the required mechanical characteristics for the belt and, in particular, substantially contribute to the modulus of the belt itself ensuring the maintenance of the performance of the belt in the course of time.
The cords are generally formed by twisting high-modulus fibres several times, and are subsequently treated with compounds adapted to increase the compatibility of the fibres with the body compound that surrounds the cords, for instance elastomeric latexes that serve as “adhesives”.
Finally, the body compound allows connecting the various elements and must have appropriate hardness and ensure that the various elements forming the belt itself synergically contribute to the final performance of the belt itself.
The known body compounds contain one or more elastomeric materials possibly enriched with fibres to increase the hardness thereof. In particular, a body compound containing copolymers such as hydrogenated acrylonitrile butadiene or HNBR is generally used for toothed belts. For poly-V belts, instead, a common solution in the art consists in the use of a body compound containing a copolymer, (optionally a diene) ethylene-propylene, referred to as EP(D)M.
However, the belts of the prior art have a considerable tendency to break when used in direct contact with lubricating oil, for instance in applications which provide that the belt is in direct contact with oil or immersed, at least partially, in an oil bath.
Currently, many drive systems use gears or chains instead of drive belts, however they are in general more complex to produce and are noisier. Furthermore, the replacement with a belt is desirable as it ensures a greater meshing precision, as well as implying lower costs.
For these reasons, chains and gears are desirably replaced by drive belts without having to carry out any other modification in the drive system as a whole, and therefore, a toothed belt will necessarily operate in contact with oil or even partially immersed in high temperature oil possibly mixed with fuel.
In such conditions, in particular, the body of the belt has a tendency to swell, which is due to the absorption of oil by the elastomeric compound and determines a considerable increase in the frequency of failure.
Patent EP1735543 suggests to improve the behaviour of belts in duration tests imposed by car manufacturers, by using belts the body of which comprises an elastomeric compound formed by a copolymer obtained from a diene monomer and a monomer containing nitrile groups, and which further comprises a fabric coating the working surface of the teeth and which is coated by a resistant elastomeric layer comprising a fluorinated plastomer and a curing agent.
Although encouraging results have been obtained thereby, solutions are continuously sought to increase the chemical-physical compatibility between the different materials forming the different components of the drive belt and between such materials and the environment in which the drive belt works, with the aim of obtaining an appropriate duration of the life of the drive belt.